Steps to Replicate?:
- Get to that screen.
- Climb up one screen.
- Fall down one screen while the other Frost Hammer Joe is firing.
- What.

I've actually gotten this to happen a few times. Similar experience. Wasn't sure of the reason it was happening and died before I could snag a screenshot each time. I have had it happen on multiple occasions. So can reiterate that this is a thing.